1. Field of the Invention
The present invention relates to sheet metal bending presses, and, more particularly, to a device for continuously measuring the angle of a sheet metal fold, as it is produced in a bending press between a die block and a penetrating press head.
2. Description of the Prior Art
It is common practice to use a sheet metal bending press for the production of shapes of various kinds from sheet metal. Such a press is generally equipped with a die block and a cooperating press head. The die block is preferably of quadrangular shape, so that it can be clamped onto the press table in any one of four different rotational positions. On its four flat sides, the die block may have one or more V-shaped longitudinal grooves of differing opening angle and/or differing width and depth. The head is normally composed of a number of longitudinally contiguous head sections of selected unequal lengths, all the head sections having the same elevational outline, i.e. identical cross-sectional shapes.
In practice, it is desirable for a sheet metal bending press to have several interchangeable heads of differing cross-sectional shape, the differences relating to (a) the lip angle defined between the two bending flanks on opposite sides of the bending edge, and/or (b) the radius at the bending edge, and/or (c) the adjoining shape of the head contour, up to a common attachment profile by which the head is attached to the press ram. The specific choice of the die block or of a particular bending groove and of the head shape depends primarily on the shape for the sheet metal workpiece to be bent, on the gauge of the sheet metal used, and/or on its deformation characteristics.
The actual bending process which takes place when a workpiece is shaped in such a bending press is not a forming process of the kind in which the sheet metal part is firmly clamped between a die and a punch, but it is more like a free-form bending process in which the sheet metal is simply pressed against the normally rounded edges of the V-groove in the die block, as the bending edge of the head is forcibly lowered into the V-groove to such a depth as is necessary to obtain the desired fold angle. The sheet metal thereby undergoes a bending action under the bending edge of the head, and the result is not only a permanent plastic deformation, but also a more or less extensive elastic deformation. Because of this elastic deformation, the fold flanks will exhibit a certain amount of springback, and a corresponding increase in the fold angle, as soon as the head pressure is released.
In order to attain the desired nominal fold angle, it is therefore necessary to bend the sheet metal workpiece beyond the nominal value of the fold angle by the amount which corresponds to the anticipated springback. The initial adjustment of the bending press, therefore, requires that, following the first bending operation, the sheet metal workpiece be removed and its fold angle measured. The workpiece then needs to be returned into the bending press, to be bent deeper by the required amount. This procedure is cumbersome and it may be difficult in the case of workpieces of large dimensions and/or considerable weight. Furthermore, it is subject to the risk that the workpiece is not replaced precisely into its previous position in relation to the bending tools, so that the subsequent bending operation will result in ruining of the workpiece. Of course, a workpiece which has been bent too deeply in the first bending operation is thereby already ruined. On the other hand, it is possible that even in a bending press which has been properly adjusted for a production run, inconsistant bending results may be obtained, due to changes in the plastic deformation of the sheet metal, for example, when the deformation resistance of the sheet metal changes, or when the sheet metal gauge varies, or also, when the grain orientation in the sheet metal is not the same for all workpieces. In the event of such inconsistant bending results, it will be necessary to subject the defective workpieces to a second bending operation or to reject them altogether.
By way of a remedy to the above-mentioned problems, it has already been suggested to equip such a sheet metal bending press with a device which provides a measurement of the fold angle of the workpiece, while the latter remains in place on the table of the bending press.
One such fold angle measuring device is disclosed in German Offenlegungsschrift (Publ. Appln.) No. 28 03 375. It features a special prism-shaped die block with a single V-shaped groove on its flat upper side. At a point along this groove is arranged a vertical bore of a diameter which is smaller than the width of the groove. In it is slidably arranged a cylindrical measuring plunger with a compression spring urging the plunger upwardly. The upper end portion of the plunger has a V-groove with flanks at the same angle and identically rounded as the flanks of the V-groove in the die block. The lower portion of the measuring plunger is a push rod which extends downwardly from the die block, reaching into a second prism-shaped body which is arranged below the die block, in alignment with the axis of the measuring plunger. There, the push rod of the measuring plunger cooperates with a rack-and-pinion drive, transmitting its vertical movements to the rack which rotates the pinion and a connected rotary indicator. Depending on the particular construction of this indicator, it is thus possible to read the distance of vertical displacement of the measuring plunger in relation to the die block or to directly determine the corresponding fold angle of the workpiece. The fold angle is a function of the vertical difference in the levels at which the outer bending edges of the die block and the outer edges of the measuring plunger contact the inclined flanks of the folded sheet metal workpiece.
The measuring plunger of this device has to have the same V-groove outline as the die block itself. Therefore, the use of this device is restricted to a particular die block and a change of tools implies a corresponding change of the fold angle measuring device. The fact that with each change in the thickness and/or material characteristics of the sheet metal stock a different V-groove profile is required, even when the fold angle remains unchanged, means that each of these grooves requires a different die block with a separate fold angle measuring device. This reflects itself in high investment costs for tooling and in the need for a large tooling inventory.
The described fold angle measuring device has the additional shortcoming that, because of its arrangement in the groove of the die lock, its measuring plunger and guide parts are exposed to dirt and foreign particles which are being wiped from the sheet metal panels as they are dragged over the die block. This dirt then accummulates in the groove and around the measuring plunger. Consequently, the measuring accuracy and operational reliability, and ultimately the longevity of such a measuring device depend to a great extent on the care and frequency with which the die block and the measuring plunger are cleaned. This means that, as soon as dirt or foreign particles have penetrated the gap between the measuring plunger or its push rod and the cooperating guide bores, these parts must be dismantled for cleaning. This procedure is difficult and time consuming, and if performed on the press table, represents down time for the sheet metal bending press.
Another fold angle measuring device is disclosed in German Offenlegungsschrift No. 20 44 199, which suggests the arrangement of a small sensing carriage on the upper side of the workpiece, in the vicinity of the bending edge of the press head, and which, by following the angular movement of the sheet metal during the bending operation in parallel alignment therewith, transmits this angular movement to a rotary potentiometer to which it is connected by means of a parallel linkage. This parallel linkage has two vertically oriented longitudinal links joined by two transverse links of which the lower one is the sensing carriage itself. The proximate vertical link is restrained to a vertical orientation by means of a longitudinal guide support, along which it moves vertically. The rotary potentiometer is arranged at the joint between the distal vertical link and the upper transverse link. The weight of the vertically movable portion of the device, assisted by a compression spring associated with the vertical guide support and a torsion spring opposite the potentiometer, act to produce a sensing pressure between the carriage and the workpiece. The entire device is carried by a vertical rod which is clamped to the press ram in a vertically adjustable position and a short intermediate rod with two adjustably clampable rod junctions, for a lateral adjustment of the position of the linkage and sensing carriage. The entire device is supported on the press ram and does not touch the press head.
The numerous joints, bearings and guides suggested for this prior art device require precision parts, implying that this device is expensive to manufacture and must be serviced regularly. The transversely oriented sensing carriage and the parallel linkage above it require considerable space in the lateral sense, even when arranged in a recess of the press head. The need for such a recess precludes any relocation of the device in the longitudinal sense of the sheet metal bend. The application of this device is therefore limited to simple, cross-sectionally straight heads, and the applicability of such a bending press is limited to simple bends on sheet metal workpieces which make allowance for the extra space requirements of this fold angle measuring device, thus precluding workpieces which require tight bends. The lateral extent of the sensing carriage also requires a comparatively large minimum distance of the fold center line from the nearest edge of the workpiece. The need for accurate parallelism of the linkage requires double joints on its four pivot points and, with the rotary potentiometer mounted on the outside of one of them, the device also occupies considerable space in the longitudinal sense of the press head. Lastly, the fact that the entire device is arranged on the outside of the press head means that there exists the risk that a sheet metal workpiece, especially one with a previously upended edge portion, may inadvertantly be pushed against the sensing carriage or the parallel linkage of the measuring device, thereby bending or otherwise damaging the latter. At the least, the device will have to be readjusted in its position.
A third prior art device for the measurement of fold angles is disclosed in the French Demande de Brevet (Publ. Appln.) No. 76 25 952. The device suggested in this publication features two parallel sensing rods arranged in the press head, at unequal lateral distances from its bending edge. The two sensing rods are oriented at a slight incline from the vertical axis of the press head cross section, extending upwardly through the latter into a gap between spacer plates attaching the press head to the press ram. In this gap, the distal sensing rod carries fixedly attached to it a vertical plate with a bellcrank whose pivot axis intersects the sensing rod axis. The proximate sensing rod engages the spring-loaded bellcrank from below, so that the two sensing rods together transmit any angular change in the sheet metal orientation to the bellcrank in the form of an identical angular change of the latter. To the axis of the bellcrank is attached a rotary potentiometer which produces an angle-related signal.
This device requires the use of special spacer plates between the press head and the press ram for the arrangement of a gap of sufficient width to accommodate the vertical plate and the potentiometer. The sensing rods and their springs are arranged in the press head itself, being not readily removable from the latter. This creates a handling difficulty for the press head, since portions of the device protrude over the press head cross section on several sides, thereby exposing the device to the risk of damage, when the press head is not attached to the press ram.